1. Field of the Invention
This invention relates to a muffler structure in which a sound absorbing material is arranged on the inside surface of an outer tube, and is retained with an inner tube made of a metal that is punched with a plurality of holes.
2. Description of Prior Art
Mufflers for motorcycles are of such a design that a sound absorbing material is installed between outer and inner tubes, and furthermore the interior of the inner tube is divided into a plurality of expansion chambers to deadening an exhaust sound. The muffler will be explained in detail by referring to the following drawing.
FIG. 6 is a schematic view of a conventional muffler structure. A sound absorbing material 102 (glass wool) is installed on the inside surface of an outer tube 101 of a muffler 100, and then is retained by an inner tube 103 made of a metal that is punched with a plurality of holes. Exhaust gas from the engine (not shown) is discharged out of the muffler 100 through the inner tube 103 as indicated by an arrow.
The inner tube 103 is formed of a punching metal as stated above, so that the glass wool 102 will appear out of punched holes 104 . . . made in the punching metal, thereby absorbing the exhaust sound.
In the inner tube 103, hot exhaust gases flow, and therefore the butt portion is jointed at both ends 105 and 106 in order that the inner tube 103 will not be opened at these ends 105 and 106 with exhaust heat. Since no clearance is formed at both ends 105 and 106, the glass wool 102 will never be scattered with the exhaust pressure.
To joint the butt portion 107, a welding operation is needed, resulting in an increase in time and manpower for manufacturing the inner tube 103. It is, therefore, hard to improve muffler productivity and to lower costs.
FIG. 7 is a developed view showing a punching metal for a conventional inner tube. The punching metal 110 has closed punched holes 104 . . . at both ends 105 and 106, so that the butt portion 107 may be welded properly at both ends 105 and 106 when forming the inner tube 104 (shown in FIG. 6).
Since the punched holes 104 . . . at both ends 105 and 105 of the metal that is punched with a plurality of holes 110 are needed to be closed, commercial punching metal is not usable for the inner tube 103 because the punched holes 104 . . . at both ends 105 and 106 of the commercial punching metal are left open.
Therefore, there is a necessity to manufacture special metal that is punched with a plurality of holes for the inner tube 103, which will be a factor responsible for an increase in the cost of the muffler.
It is, therefore, an object of the invention to provide a muffler structure capable of lowering the cost of the muffler.
In view of the above-described problems, this invention provides in a first aspect a muffler structure in which a sound absorbing material is laid on the inside surface of an outer tube; the sound absorbing material is retained by an inner tube made of a metal that is punched with a plurality of holes. The inner tube is separated into a plurality of expansion chambers. Exhaust gas from an engine is directed into one of the expansion chambers by an inlet pipe. The exhaust gas is discharged out after passing through the other expansion chambers one after another through a connecting pipe. In the muffler structure, the inner tube is made of an approximately flat punching metal by rolling until its opposite edges are butted to each other, and is fitted in the outer tube, with the butt portions not jointed, so that the butt portions will come behind one of the connecting pipes when viewed from the inlet pipe side.
The inner tube, therefore, is inserted into the outer tube in such a manner that the butt portion of the inner tube will come behind one of the connecting pipes when the butt portion of the inner tube is viewed from the inlet pipe side. Therefore the radiant heat of the inlet pipe can be shielded by the connecting pipe, thereby enabling to prevent the direct transfer of the radiant heat of the inlet pipe to the butt portion of the inner tube. Therefore, no large clearance will occur at the butt portion that is not joined.
Furthermore, because it is unnecessary to joint the butt portion of the punching metal, a metal that is punched with a plurality of holes having open punched holes at both end portions is usable. Therefore, a commercial punching metal may be used after cutting to a predetermined shape, whereby a metal that is punched with a plurality of holes for the inner tube is obtainable at a low cost.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.